The Hel-X 13 BioMedia for the BioMaster 250, 350, or 600 includes a replacement BioMedia allotment. The allotment contains 0.3 gallons of Hel-X BioMedia. Hel-X BioMedia has a large surface area across this suspending media form which is ideal for biological filtration. It is best to only replace the BioMedia when you do a complete tank restart as to not disturb the beneficial biological load.